This invention relates to the propagation of acoustic waves and, more particularly, to the transmission of high frequency acoustic waves known as phonons.
Acoustic waves or sound waves result from the vibrations of atoms or molecules and at audible frequencies (lower than about 20 KHz) can be generated in a variety of ways: as human speech by the movement of our vocal chords, as instrumental music by forcing air through a trumpet, and as noise by pounding on a table top. Sound waves at these frequencies propagate reasonably well through the atmosphere as evidenced by the fact that the waves can be heard from afar. But, at higher ultrasonic frequencies (about 1 MHz), where sound waves are inaudible, the atmosphere is a poor medium for propagation; ultrasonic waves are rapidly dissipated in the gases of the air and can be made to travel appreciable distances only in condensed media such as liquids and solids.
On the other hand, ultrasonic waves are widely exploited for imaging; that is, for taking acoustic pictures by making sound waves, instead of light waves, incident on the object being imaged. These ultrasonic cameras are currently available for the study of underwater objects, internal structural features in materials and organs inside the human body. However, the wavelength of an ultrasonic camera is typically in the millimeter range which limits the resolution of the image to features no smaller than that size. However, the next generation of ultrasonic imaging apparatus, the acoustic microscope, offers hope of dramatically increasing resolution so that features of micrometer size can be "seen" acoustically.
As described by C. F. Quate in Scientific American, Vol. 241, No. 4, p. 3 (1979), the acoustic microscope is an experimental device which operates at acoustic frequencies in the vicinity of 1 GHz and, in the most advanced versions, at wavelengths measured in micrometers and fractions of a micrometer. At these wavelengths the acoustic microscope is comparable in resolution to the optical microscope, but, as Quate points out, resolution is only one parameter of the comparison, contrast is another. Acoustic waves and optical (light) waves are reflected and absorbed differently by the same object, making the acoustic image quite different in microscopic information content from the optical image. Quite plainly, the acoustic microscope "sees" microscopic features which the optical microscope cannot.
The basic physical phenomenon underlying the acoustic image is the change in velocity of sound waves as they cross an interface between two different materials. Quate notes that the velocity of sound waves can decrease by a factor of ten in traversing a solid-liquid interface. Indeed, the nature of the materials (i.e., their acoustic impedances) as well as the quality of the interface (i.e., the number of defects it contains) can even more drastically reduce the velocity of the waves and may even convert them to heat.
This problem is particularly acute at even higher acoustic frequencies in the hundreds of gigahertz range. At these enormously high frequencies, the sound waves are referred to as phonons, which may have extremely short wavelengths in the 10 Angstrom to several hundred Angstrom range.
Phonons are critically sensitive to the quality of an interface they are required to traverse. In this regard, an extensive study of the loss processes in phonon generation and detection was reported by H. J. Trumpp et al in Z. Physik, Vol. B 28, p. 159 (1977). They formed a first superconducting tunnel junction (STJ) on one surface of a silicon substrate to generate 280 GHz phonons in the substrate and formed a second STJ on another surface of the substrate to detect the phonons. They found that the total phonon loss was about 90 percent; that is, only about 10 percent of the phonons generated at the first STJ were above to travel through the substrate to the second STJ; the remainder was converted to heat. In addition, their measurements indicated that the main sources for phonon losses were localized at the boundaries of the tunnel junctions to the substrate. These boundaries were interfaces between the base superconducting electrodes of the STJs and the substrate.
With such phenomenally high losses one would expect that a phonon device having a plurality of interfaces would have virtually no utility as a practical matter. For example, if the phonon transmission through a single interface is only 10 percent, then after traversing N such interfaces the phonon signal would be 10.sup.-N of its original value. For N=10, the signal level would be 10.sup.-10 times its original value, making detection of such low levels virtually impossible. Even if the transmission through the interface were twice as good, 20 percent, after traveling through ten interfaces, the phonon signal would be down to (2.times.10.sup.-1).sup.10 .about.10.sup.-7 of its original value--still an extremely low level. It is apparent, therefore, that the prior art, as typified by the Trumpp et al. article, suggests that a plurality of interfaces is something to be avoided in phonon devices.